Magnetic recording media having as a magnetic layer a ferromagnetic-metal thin film suitable for high-density recording have been practically used as magnetic tapes, hard disks, etc. Since such the magnetic recording media having a ferromagnetic-metal thin film as a magnetic layer can easily attain a high magnetic energy and simultaneously have exceedingly high surface smoothness, they have an advantage of having a reduced spacing loss and high electromagnetic characteristics.
However, too high a degree of surface smoothness not only results in an increase in real contact area but leads to adsorption of the lubricant present on the magnetic-layer surface onto a mating member, making it very difficult to ensure running durability. Because of this, the running durability of those conventional recording media is ensured at a slight sacrifice of electromagnetic characteristics by forming adequate surface roughness on the substrate, which roughness formation is accomplished by coating on the base film surface a coating solution comprising fine particles and a binder in the case of magnetic tapes, or by mechanically grinding the substrate in the case of hard disks.
In JP-A-57-8921 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") is disclosed a magnetic recording medium produced by forming an SiO.sub.2 or glass film by sputtering on a substrate coated with a metal or oxide film and then forming a magnetic oxide film on the SiO.sub.2 or glass film by sputtering, in which the obtained recording medium has an improved magnetic characteristics. In JP-A-57-52646 is disclosed a magnetic recording medium produced by forming an SiO film by vapor deposition directly on a polymer molding as a substrate and forming at least a magnetic metal film on the SiO layer, in which the obtained recording medium has an improved life. Further, JP-A-59-207422 discloses a magnetic recording medium produced by forming on a film substrate an undercoating layer having a thickness of 0.2 .mu.m or larger and containing fine inorganic particles with an average particle diameter of from 3 to 50 nm and then forming a thin ferromagnetic-metal film thereon, in which the obtained recording medium has an improved mechanical tape strength.
The surface roughness ideal for the attainment of both a high degree of electromagnetic characteristics and a high degree of running durability is one in which projections having almost the same height are densely present on the medium surface.
In recent magnetic recording media, however, the height of such the projections tends to become smaller gradually because higher electromagnetic characteristics have come to be required due to the strong desire for high-density recording. In such the magnetic recording media, low projections having a height of about 10 to 20 nm should be evenly formed, but the formation of such the projections is difficult.
In the case of magnetic tapes, for example, the fine particles to be applied to the surface of a nonmagnetic support are required to be ultrafine particles having a particle diameter of about 10 nm as described above. Even though such the ultrafine particles can be produced as a monodisperse system, it is extremely difficult to apply the ultrafine particles to a nonmagnetic support without causing aggregation of the ultrafine particles, which have a high surface energy. Even if this aggregation occurs locally, the resulting aggregates are mostly present at a density sufficient to form undesirable spaces between a head and the magnetic recording medium. Thus, the use of ultrafine particles only gives recording media having electromagnetic characteristics lower than those expected from the diameter of the particles applied.
On the other hand, in hard disks also, it is difficult to evenly form low projections as described above by a mechanical means.
Under these circumstances, the present inventors made investigations in which fine particles having a particle size of about 20 to 30 nm, which is the ordinary size at present, were used to form on the surface of a nonmagnetic support an undercoating layer having an increased thickness so as to lessen the influence of particle size as much as possible.
However, the undercoating layer having an increased thickness has a problem that the so-called "thermal damage phenomenon" occurs in which the magnetic layer being formed by sputtering or vapor deposition develops large projections probably because the binder contained in the undercoating layer changes during the formation of the magnetic layer.